A first blush, an Andrew Romanoff-Michael Bennet primary for U.S. Senate looks more like the “Patty Duke Show” than an old-fashioned slugfest.

Sing along if you remember the words (I cheated and Googled them): “But they’re cousins, identical cousins and you’ll find, they laugh alike, they walk alike, at times they even talk alike. You can lose your mind, when cousins are two of a kind!”

But only Democrats are losing their minds this time. Republicans are salivating at the thought of a primary. As one Republican activist told me, it’s usually the GOP that has circular firing squads.

But a Romanoff entry into the race may not spell doom at all. It could be Bennet’s biggest gift since he was appointed to the seat by Gov. Bill Ritter in early January.

Yes, a primary could be costly, politically and financially, for both.

But Romanoff and Bennet are so similar, personally and ideologically, it may be hard for those third- party attack groups to scratch up the dirt for a nasty attack ad that would stick.

Even Patty Duke and her cousin had bigger differences.

Both guys are part of Generation Obama. Like the president, who is 48, the two Democrats, at 44, are a tad too old to be Gen Xers and too young to be baby boomers.

They’re both center-left, white Yalies with law degrees. Romanoff did his undergrad work at Yale and was editor of the Yale Daily News. Bennet earned his law degree from Yale Law School and served as editor of the Yale Law Journal.

They’re both non-native species to Colorado, and their roots here, however shallow, are only in Denver. Neither would look terribly comfortable in a bolo tie.

But Romanoff knows, by name, the party activists in far-flung places, and many are still smarting that Ritter passed him over for the seat. Long the boy wonder of the state Democratic Party, Romanoff no doubt will excite those insiders who cast a suspicious eye at Bennet, a guy who didn’t pay his dues by attending umpteen rubber chicken dinners in Lamar or Limon.

Romanoff could win top line at the state assembly, where party activists show their strength. But that only means his name would appear first on the primary ballot. Activists elevated Mike Miles and Bob Schaffer at party assemblies in 2004, and neither has the title of “senator” today.

To win a primary, Romanoff will have to attack Bennet from the left, where he is vulnerable. If Bennet tacks too far to port to get through the primary, he could have a tough time winning the general election.

But Romanoff’s candidacy is more likely to push Bennet right into the middle, where he has been trying to stake his ground anyway.

Some Democrats fear Romanoff might play the role of Gene Nichol, who, in a 1996 Democratic Senate primary, tarred Tom Strickland with the label “lawyer-lobbyist,” which Republicans used quite effectively in the general election. Twice.

It’s a real fear, but it hardly works for a lawyer-lawmaker to call out another lawyer-lawmaker.

Romanoff’s biggest challenge will be raising cash. Bennet already has a war chest of more than $3 million, and if Romanoff begins to gain traction, Bennet can write out a hefty check to his own campaign to blanket the airwaves with ads, ensuring that he, not Romanoff, defines himself and his message.

By not coming down with a decisive “yes” or “no” on the Employee Free Choice Act, Bennet has let Republicans and others shape that debate. A Romanoff candidacy would force Bennet to clarify and more fully develop his positions on issues, and it might even thicken up his thin skin.

Bennet, who has never won an election, could emerge as a battle-tested winner, ready to knock off any Republican comer.

All thanks to his cousin.

Editorial page editor Dan Haley can be reached at dhaley@denverpost.com.